Browsing by Author "Gurgiolo, C."
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- ItemCharacteristics of the Taylor microscale in the solar wind/foreshock: magnetic field and electron velocity measurements(EGU, 2013-11-22) Gurgiolo, C.; Goldstein, Melvyn; Matthaeus, W. H.; Viñas, A.; Fazakerley, A. N.The Taylor microscale is one of the fundamental turbulence scales. Not easily estimated in the interplanetary medium employing single spacecraft data, it has generally been studied through two point correlations. In this paper we present an alternative, albeit mathematically equivalent, method for estimating the Taylor microscale (λT). We make two independent determinations employing multi-spacecraft data sets from the Cluster mission, one using magnetic field data and a second using electron velocity data. Our results using the magnetic field data set yields a scale length of 1538 ± 550 km, slightly less than, but within the same range as, values found in previous magnetic-field-based studies. During time periods where both magnetic field and electron velocity data can be used, the two values can be compared. Relative comparisons show λT computed from the velocity is often significantly smaller than that from the magnetic field data. Due to a lack of events where both measurements are available, the absolute λT based on the electron fluid velocity is not able to be determined.
- ItemDirect observations of the formation of the solar wind halo from the strahl(EGU, 2012-01-17) Gurgiolo, C.; Goldstein, Melvyn; Viñas, A. F.; Fazakerley, A. N.Observations of a continual erosion of the strahl and build up of the halo with distance from the sun suggests that, at least in part, the halo may be formed as a result of scattering of the strahl. This hypothesis is supported in this paper by observation of intense scattering of strahl electrons, which gives rise to a proto-halo electron population. This population eventually merges into, or becomes the halo. The fact that observations of intense scattering of the strahl are not common implies that the formation of the halo may not be a continuous process, but one that occurs, in part, in bursts in regions where the conditions responsible for the scattering are optimum.
- ItemFirst measurements of electron vorticity in the foreshock and solar wind(EGU, 2010-12-21) Gurgiolo, C.; Goldstein, Melvyn; Viñas, A. F.; Fazakerley, A. N.We describe the methodology used to set up and compute spatial derivatives of the electron moments using data acquired by the Plasma Electron And Current Experiment (PEACE) from the four Cluster spacecraft. The results are used to investigate electron vorticity in the foreshock. We find that much of the measured vorticity, under nominal conditions, appears to be caused by changes in the flow direction of the return (either reflected or leakage from the magnetosheath) and strahl electron populations as they couple to changes in the magnetic field orientation. This in turn results in deflections in the total bulk velocity producing the measured vorticity.
- ItemObservations of electron vorticity in the inner plasma sheet(EGU, 2011-09-01) Gurgiolo, C.; Goldstein, Melvyn; Viñas, A. F.; Matthaeus, W. H.; Fazakerley, A. N.From a limited number of observations it appears that vorticity is a common feature in the inner plasma sheet. With the four Cluster spacecraft and the four PEACE instruments positioned in a tetrahedral configuration, for the first time it is possible to directly estimate the electron fluid vorticity in a space plasma. We show examples of electron fluid vorticity from multiple plasma sheet crossings. These include three time periods when Cluster passed through a reconnection ion diffusion region. Enhancements in vorticity are seen in association with each crossing of the ion diffusion region.
- ItemA phase locking mechanism for nongyrotropic electron distributions upstream of the Earth's bow shock(AGU, 2005-06-11) Gurgiolo, C.; Goldstein, Melvyn; Narita, Y.; Glassmeier, K.-H.; Fazakerley, A. N.Observations of nongyrotropic electron distributions in the region upstream of the Earth's bow shock suggest that there exists a mechanism to lock in their phase, otherwise they would rapidly gyrophase mix into a ring-beam distribution. Measurements by the Plasma Electron and Current Experiment (PEACE) on the Cluster spacecraft have provided a way of determining the rotational period of a nongyrotropic electron distribution. For the time period studied, the rotational period is found to be ≈0.5 Hz, significantly lower than the local Larmor frequency but in line with the frequency of waves observed in the local magnetic field. Detailed wave analysis has revealed that the waves are most likely ordinary right-hand whistler waves. The conclusion is that the waves provide the necessary phase locking mechanism. It is not clear whether those waves are generated by the nongyrotropic distribution or are produced by other means and then cause the observed nongyrotropy.
- ItemSolitary Electromagnetic Pulses Detected with Super-Alfvénic Flows in Earth’s Geomagnetic Tail(APS, 2007-06-27) Parks, G. K.; Lee, E.; Lin, N.; Mozer, F.; Wilber, M.; Dandouras, I.; Rème, H.; Lucek, E.; Fazakerley, A.; Goldstein, Melvyn; Gurgiolo, C.; Canu, P.; Cornilleau-Wehrlin, N.; Décréau, P.Solitary nonlinear ( δB=B >> 1) electromagnetic pulses have been detected in Earth’s geomagnetic tail accompanying plasmas flowing at super-Alfvenic speeds. The pulses in the current sheet had durations of ∼5 s, were left-hand circularly polarized, and had phase speeds of approximately the Alfven speed in the plasma frame. These pulses were associated with a field-aligned current Jₗₗ and observed in low density (0:3 cm⁻³), high temperature (Te ∼ Ti ∼ 3 x 10⁷ K), and β ∼ 10 plasma that included electron and ion beams streaming along B. The wave activity was enhanced from below the ion cyclotron frequency to electron cyclotron and upper hybrid frequencies. The detailed properties suggest the pulses are nonlinearly steepened ion cyclotron or Alfven waves.
- ItemWhistler Waves Driven by Anisotropic Strahl Velocity Distributions: Cluster Observations(AIP, 2010-03-25) Viñas, A. F.; Gurgiolo, C.; Nieves‐Chinchilla, T.; Gary, S. P.; Goldstein, MelvynObserved properties of the strahl using high resolution 3D electron velocity distribution data obtained from the Cluster/PEACE experiment are used to investigate its linear stability. An automated method to isolate the strahl is used to allow its moments to be computed independent of the solar wind core+halo. Results show that the strahl can have a high temperature anisotropy (T⊥/ T ₗₗ ≳ 2) This anisotropy is shown to be an important free energy source for the excitation of high frequency whistler waves. The analysis suggests that the resultant whistler waves are strong enough to regulate the electron velocity distributions in the solar wind through pitch‐angle scattering.